The invention is in the field of impellers and propellers, and in particular, pertains to the construction of an impeller blade for a wind turbine, although the scope of the invention is not limited to this use, as it might be useful in other types of impellers, and perhaps propellers as well.
The blade construction maximizes the strength versus the weight of the blade, as weight is a major consideration in large, rapidly rotating wind turbine rotors. The blade is designed to enable the entire length of the blade to be externally configured to define an airfoil in cross-section and every point along its length, particularly at its root, requiring that the internal reinforcement structure be sufficiently narrow in diameter that the airfoil cross-section at that region need not be of oversized height, destroying the airfoil shape and thus reducing the efficiency of the rotor.
Wind turbines must endure enormous destructive forces during their operation. Because wind is a natural force and cannot be controlled, the wind turbine must accommodate and survive wind conditions varying from no wind at all to gale force winds. Every wind turbine must have some mechanism to accommodate high winds. Either the turbine is shut down altogether, or a brake mechanism is incorporated to prevent the turbine from going too fast, and tearing itself apart. Any unbraked mechanism which is reasonably efficient at ordinary wind velocity would tear itself apart in high winds if some mechanism for stopping the rotor or governing its speed were not used.
Although the use of the instant blade goes beyond its immediate application, it was designed for use on a wind turbine with adjustable-pitch blades, that is, blades having a mechanism to vary the blade pitch at all operable wind speeds to maintain a steady 120 rotor revolutions per minute.
Design considerations of the blade and its rotors naturally include a careful balancing of the rotor to reduce vibration, and construction of the rotor blades such that they are as lightweight as possible to reduce the strain on the tower, and so that they are not subject to resonance and harmonic vibration at their operating speeds. Also, the blades must have considerable strength to endure the buffeting of the winds and the stress they experience being constantly exposed to natural forces.
The most stressed region of the blade is at its root, where the moment arm of the blade is quite high and could snap the blade off its rotor if the root, and the junction of the root to the rotor, were not extremely strong. A typical lightweight rotor blade is made as a hollow fiberglass and resin skin to maximize strength while minimizing weight. The internal support structure for the external skin, particularly where the blade mounts to the rotor, must be very well engineered. Otherwise, the support structure will either be too large to permit the exterior of the blade to assume a true airfoil cross-section adjacent the root, or if it is a solid bar, for instance, weight of the blade would be increased more than is absolutely necessary to secure the required strength.
Another design consideration of some importance in the construction of blades for wind turbines is the fact that because the mechanism takes such abuse from the elements and is thus subject to metal failure, it is desirable that the entire internal support be such that a failure in one portion of the steel support will not propagate throughout the entire support structure, causing the blade support to fail, the blade to fly off, and the wind turbine to "crash."